Waste of memory storage

Try as I may, I keep forgetting the kanji for 遺伝子 (いでんし: “gene, genetics”)…

On the other hand I have absolutely no trouble remembering words like 居合い (いあい: “act of drawing one’s katana, killing with a single stroke, shaking out the blood and sheathing the katana back, as one single movement” — yes, it means all that; no, I am not kidding) after seeing them used exactly once.